This invention relates to an activated sludge wastewater treatment process for removing phosphate from BOD-containing wastewater to obtain a substantially phosphate free effluent.
In the conventional activated sludge system in use today, wastewater is subjected to the usual screening and pretreatment steps, e.g., primary sedimentation, then mixed with recycled activated sludge to form a mixed liquor which is subjected to aeration with an oxygen-containing gas in an aeration zone. During aeration of the mixed liquor, the bacteria (microorganisms) present in the activated sludge cause the aerobic decomposition of solids and a high degree of BOD removal is achieved.
Phosphates, which are present in organic wastes and detergents, escape conventional wastewater treatment processes and are released with the effluent into natural water resources, e.g., lakes, rivers and streams. These phosphates result in over-fertilization or eutrophication of waters causing unsightly algal blooms and serious pollution problems.
Phosphorus impurities are normally removed from wastewater by chemical treatment (precipitation). Three basic chemical treatment procedures have been proposed for removing phosphorus in association with activated sludge. These procedures include pre-treatment by precipitation and removal upstream of the activated sludge system, post-treatment by precipitation and removal downstream of the activated sludge system, and combined treatment and removal in situ of the activated sludge system.
Pre-treatment is conveniently accomplished by adding precipitating chemicals upstream of or within the primary clarifier so that the chemical sludge and the primary (organic) sludge can be separated together. Post-treatment is accomplished by adding precipitating chemical to the effluent from secondary treatment in a separate mixing tank. The resultant chemical sludge is then separated in an additional clarifier. Combined treatment is practiced by adding phosphorus-precipitating chemicals directly to the mixed liquor in the activated sludge treatment step and thereby removing both carbonaceous matter and phosphorus simultaneously and in the same equipment.
A widely used method for chemically precipitating phosphorus pollutants is by the addition of a metal compound such as aluminum sulfate (alum) or ferric chloride. The phosphorus is precipitated as a result of a chemical reaction with the precipitating agents. Calcium oxide (lime) has also been widely employed to precipitate phosphorus, but the precipitation is pH dependant rather than occuring as a stoichiometric reaction.
The three basic chemical techniques for phosphorus removal have several disadvantages. In phosphorus precipitation, there are other competing chemical reactions within the treatment process which consume a portion of the chemical additive, and dosages substantially in excess of the stoichiometric ratio with phosphorus must be applied in order to obtain desired removals. This effect is especially observed in the pre-treatment and combined-treatment approaches. These techniques also require the treatment of very large volumes of low phosphorus containing liquid which adds to the chemical precipitant requirements, especially when lime is used. Finally, needed large chemical doses result in the generation of large volumes of chemical sludge and produces an additional disposal problem. This is especially acute in the pre-treatment and combined treatment techniques where the chemical sludge contains a large fraction of biologically active material. Moreover, the large volume of inerts that accumulate in the combined treatment approach (e.g., 50% of the total solids) and the necessary increase in sludge wasting required to limit such accumulation may hinder the carbonaceous removal efficiency of the activated sludge system.
As an alternative to pure chemical removal techniques, Levin U.S. Pat. No. 3,236,766 describes a process for biologically removing phosphates from a wastewater stream. Raw or primary treated sewage is aerated with air or enriched oxygen gas together with recycled activated sludge for sufficient duration (i.e., 1 to 8 hours) to cause sludge bacteria present to take-up phosphates in excess of their requirements for growth (luxury uptake). A phosphate-enriched sludge is then separated from a phosphate-depleted effluent. A portion of the phosphate-enriched sludge may be wasted and subsequently converted into fertilizer. The remaining phosphate-enriched sludge is passed to a combination stripper and sludge thickener where it is adjusted to an acidic pH and maintained in the anaerobic condition. The acidic pH and anaerobic condition cause a significant quantity of phosphate to resolubilize and a phosphate-rich liquid is separated from a lower phosphate content sludge. The latter is returned to the aeration zone for the removal of influent phosphates while the phosphate-rich liquid is chemically treated to precipitate phosphate and may then be discharged. The Levin process permits high levels of phosphate removal from wastewater by insuring high phosphate uptake by sludge solids in aeration, subsequently stripping the phosphates from the solids into the associated liquid by a combination of anaerobic and acidic treatment and chemically treating the phosphate-rich liquid separated from the solids. This process effectively avoids the problems that confront chemical phosphorous removal systems. Specifically, the operating costs associated with the chemical precipitant and the quantity of chemical sludge produced have been substantially reduced since a much reduced volume of liquid needs treatment for phosphate removal.
The Levin process requires a considerable time for the released soluble phosphate in the anaerobic sludge, and particularly in the settled sludge in the lowermost section of the stripping zone, to migrate out of the settled sludge layer and into the supernatant liquor in the stripping zone. Such slow migration is a consequence of physical obstruction to the diffusional flow of released phosphate by the thickened sludge solids as well as inherent equilibrium limitations to the mass transfer process. Under such conditions, if the sludge is withdrawn from the stripping zone and recycled to the aeration zone before a sufficient amount of the soluble phosphate is transferred to the supernatant liquor, an excess amount of soluble phosphate is recycled to the aeration zone and the phosphate removal efficiency of the overall process is undesirably lowered.
As a solution of this problem, Matsch et al U.S. Pat. No. 4,042,493 discloses a process for countercurrently stripping soluble phosphates from the phosphate-lower sludge in the stripping zone. In the Matsch et al process, a low phosphate and low solids stripping medium is introduced into the lower section of the stripping zone for upflow through at least part of the settling solids introduced to the stripping zone as phosphate-enriched sludge. In this manner, the phosphate released from the settling sludge solids is transferred to the upflowing liquid to provide the phosphate-rich liquid. As a result, the phosphate stripping zone may be effectively operated in a countercurrent extraction mode to obtain higher removals of phosphate than economically possible with the methods of the prior art.
Matsch et al process is an improvement on the original Levin process but is more complicated from the control standpoint, requires additional equippment, higher power costs and does not eliminate the need for chemical treatment to remove phosphates.
Accordingly, it is an object of this invention to provide an improved process for reducing the phosphate content of phosphate-containing wastewater in an activated sludge wastewater treatment system, which is less complex, uses less equipment, requires less power and eliminates the need for chemical treatment to remove phosphates.
Other objects and advantages will be apparent from the ensuing disclosure and claims.